In a conventional diesel engine, the maximum internal cylinder pressure (i.e., the maximum generated pressure in a cylinder) is at a very high level, the maximum internal cylinder pressure, particularly in the idle state, is far higher than in a gasoline engine with a throttle in an intake system ("Isuzu Technical Papers", Vol. 99, p. 85).
As a result, for the same difference of engine torque fluctuation and vibration, the noise and vibration of a diesel engine are very marked in comparison to a gasoline engine in the idle state.
A technique for reducing noise in the idle running state of a diesel engine is disclosed in Japanese Patent Publication Tokkai Sho 56-101031. In this invention, a shutter is provided in an intake air passage of the engine, this shutter being closed in the idle running state. This reduces the intake air amount and decreases the pressure in the compression stroke. By decreasing the compression pressure, the maximum pressure generated when fuel is burnt also decreases, so noise in the idle running state is therefore mitigated.